You can apply crossfades for audio transitions between
clips. An audio fade is analogous to a video transition. For a crossfade,
you add an audio transition between two adjacent audio clips on
the same track. To fade in or fade out, you add a crossfade transition
to either end of a single clip. Premiere Pro includes three types
of crossfade: Constant Gain, Constant Power, and Exponential Fade.

Crossfade between audio clips

If necessary, click the triangle to
the left of each track name in a Timeline panel to expand the audio
tracks that you want to crossfade.

Make sure that the two audio clips are adjacent, and
that both clips are trimmed.

Do one of the following:

To add the default audio transition, move
the current-time indicator to the edit point between the clips,
and choose Sequence > Apply Audio Transition.

To add an audio transition other than the default,
expand the Audio Transitions bin in the Effects panel and drag the
audio transition to a Timeline panel, on the edit point between
the two clips you want to crossfade.

Fade in or fade out clip audio

Make sure that the audio track is expanded
in a Timeline panel. If necessary, click the triangle to the left
of the track name to expand the audio tracks that you want to crossfade.

Do any of the following:

To fade in a clip’s audio, drag an audio
transition from the Effects panel to a Timeline panel so that it
snaps to the In point of the audio clip. You can also select the
applied transition in a Timeline panel. Then, in the Effect Controls panel
choose Start At Cut from the Alignment menu.

To fade out a clip’s audio, drag an audio transition
from the Effects panel to a Timeline panel so that it snaps to the
Out point of the audio clip. You can also select the applied transition
in a Timeline. Then, in the Effect Controls panel, choose End At
Cut from the Alignment menu.

Use any of the three types of audio crossfade transitions
to fade in or fade out.